Nuclear magnetic resonance (NMR) spectroscopy, by which one can get atomic level information on matter, is an excellent method of analyzing the structure of a compound. The basic principle of analysis by this method is as follows: a sample placed in a static magnetic field is exposed to a radio frequency magnetic field and a response signal from the excited nuclear spin is received and analyzed. For high resolution analysis, an NMR spectrometer which has a superconducting magnet capable of generating a static magnetic field (B0) is employed. Currently, a 21.6 T (920 MHz) NMR spectrometer is available as an NMR spectrometer which is primarily intended to analyze three-dimensional protein structures.
In protein analysis, the sample volume is very small and the intensity of generated free induction decay (FID) signals is weak. Therefore, a probe for receiving FID signals must be highly sensitive. As described in Patent Document 1 (U.S. Pat. No. 5,247,256 specification), a method which reduces thermal noise by the use of a probe at a low temperature to increase the probe sensitivity has been known. In addition, the use of a superconducting material for the probe coil is also effective in increasing the sensitivity. Surface resistance in superconducting materials is two or more digits lower than that in normal metals such as copper. Therefore, the use of a superconducting material reduces the surface resistance of the probe coil and enables reception of signals with a high sensitivity. An example of the use of a superconducting probe coil is described in Patent Document 2 (U.S. Pat. No. 5,585,723 specification).
Superconductor used for the probe coil is a thin film made on a planar substrate and the film surface is arranged parallel to a static magnetic field in order to prevent deterioration in static magnetic field uniformity due to the perfect diamagnetism of the superconductor. In a typical NMR spectrometer, a static magnetic field is generated vertically (vertical type NMR spectrometer). A sample tube containing sample is inserted vertically and a substrate with superconducting thin film is disposed in a way to surround the sample tube. Hence, the superconductor-based probe coils in the prior art are of the Helmholtz type, saddle type or bird cage type. A probe coil of the bird cage type is illustrated in Patent Document 3 (Japanese Patent No. 3066359 specification). On the other hand, an NMR spectrometer (horizontal type NMR spectrometer) which generates a static magnetic field horizontally can use a solenoid probe coil. Solenoid probe coils are higher in filling factor than Helmholtz type, saddle type or bird cage type probe coils and thus higher in sensitivity.